The present invention relates to power circuits in general and, in particular, to high-voltage power converter circuits.
Many electronics applications use switching power converters to convert a DC bus voltage to an appropriate load voltage. When the DC bus voltages are sufficiently low, it may be possible to implement many functions of the converter with low-voltage components. For example, it may be desirable to regulate the output of the converter to a load by sensing the current flowing to the load. In a low-voltage converter, low-voltage components may be used to implement current sensing and feedback circuitry (e.g., oscillators, error amplifiers, etc.). These low-voltage components may typically be less expensive and smaller than their high-voltage counterparts, and may be more fully integrated into integrated circuits (“ICs”) with less expensive manufacturing processes.
In some applications, however, the DC bus voltage is relatively high. For example, certain applications may include low voltage electronics (e.g., operating at 5 or 12 volts DC), configured to receive power from mains line voltage (e.g., via a wall outlet operating at 110 or 220 volts AC). Often, conversion of the mains AC voltage to the low-voltage DC load output involves multiple stages of conversion: a first stage that converts the high-voltage AC input to a relatively high-voltage DC bus; and a second stage that converts the high DC bus voltage to a low DC output voltage. Because the second (e.g., output) stage is driven by a high-voltage DC bus, certain high-voltage circuit topologies may be used, which may limit the ability to use low-voltage components and manufacturing processes. This may, in turn, make it difficult and/or expensive to implement functionality, such as current sensing, to add to the effectiveness of the switching converter.
FIG. 1A shows a simplified schematic view of a typical “buck” switching converter 100 implemented with a high-side switch topology. As illustrated, the buck converter includes a DC source 110 that generates a positive DC bus voltage rail 102 and a negative DC bus voltage rail 104. For example, the DC source 110 is a first stage converter that converts an AC input voltage to the DC bus voltage (i.e., the difference between the positive DC bus voltage rail 102 and the negative DC bus voltage rail 104). In some embodiments, the negative DC bus voltage rail 104 is tied to ground. The buck converter is configured to step down the DC bus voltage to a lower voltage appropriate for driving a load 150.
The buck converter includes a power switch 130. In some embodiments, the power switch 130 is implemented as a P-channel MOSFET. As shown in FIG. 1A, power switch 130 is configured as a high-side switch tied to the positive DC bus voltage rail 102. A gate driver module 120 generates a switching signal (e.g., a square wave) that drives power switch 130, such that power switch 130 opens and closes as a function of a duty cycle and a switching frequency. The high-side configuration may allow the bus voltage and the load 150 to share a common ground, which may be desirable or even required by certain applications.
In each cycle, when power switch 130 is closed (e.g., power switch 130 is ON), current flows from the positive DC bus voltage rail 102, through an inductor 116, and into the load 150. As current flows through inductor 116, stored energy is built up in inductor 116 (e.g., as a function of the peak current flowing into inductor 116). In each cycle, when power switch 130 is open (e.g., the switch is OFF), the connection between inductor 116 and the positive DC bus voltage rail 102 is interrupted. A diode 114 allows the energy stored in inductor 116 to flow to the load 150, demagnetizing inductor 116. In certain embodiments, inductor 116 fully demagnetizes in each cycle while power switch 130 is open (“discontinuous” operation). In other embodiments, inductor 116 only partially demagnetizes before the next cycle begins, and power switch 130 closes again (“continuous” operation). Further, in particular embodiments, one or more capacitors 112 are provided, for example, to reduce voltage and current ripple to the load 150.
The operation of the converter may also be seen in FIG. 1B, which shows a graph 135 of two cycles of the current through power switch 130, a graph 115 of two cycles of the current through diode 114, and a graph 117 of two cycles of the current through inductor 1 16. In some embodiments, operation of the switching converter 100 is improved by increasing the ON-time of power switch 130 during each cycle. During each cycle, it can be seen from graph 135 and graph 117 that the current through inductor 116 ramps up while power switch 130 is ON, and the current through inductor 116 ramps down while power switch 130 is OFF (i.e., while current is flowing through diode 114, as shown in graph 115). It is worth noting that the current through inductor 116 never reaches zero (i.e., the graph is showing “continuous” operation), and the range between the minimum and maximum currents may be relatively small in certain applications (e.g., a five- to ten-percent variation).
It will be appreciated that this circuit topology may allow current to flow to the load 150 during both parts of each cycle, both when power switch 130 is open and when power switch 130 is closed. In some applications, this may be desirable when compared to other switching regulator techniques, for example, in relation to ripple, noise (“EMI”), and efficiency performance. Further, the high-side configuration may be implemented with low-voltage components and manufacturing processes, so long as the bus voltage is sufficiently low. For example, silicon processes may be readily available for implementing highly integrated controllers at bus voltages of up to approximately 40 to 60 volts. These integrated controllers may include desirable and/or necessary functions, such as oscillators, voltage references, error amplifiers, current limiters, thermal protection circuits, level shifters, power switches, etc. Some advanced silicon processes may even allow integration of certain demagnetization circuitry by replacing diode 114 with a MOSFET driven synchronously, which may provide more efficient and integrated solutions.
Many applications, however, desire to operate with higher bus voltages. For example, it may be desirable to design certain electronics applications to operate “off-line” (e.g., by converting a mains line voltage to a DC bus voltage). In various geographic regions, this “off line” operation may generate high bus voltages (e.g., in a range of 150 to 600 volts). In many of these high-voltage applications, it may not be possible or desirable to use low-voltage converters (e.g., buck converters) to step down the load voltage. High-voltage step-down converters, however, may not be able to use the same solutions as low-voltage applications to provide efficient and affordable integrated solutions.
Particularly, when using a high-side switch topology in a high-voltage environment, it may be difficult to implement control circuitry using low-voltage components. For example, current sensing may be more difficult in high-voltage converters. With low-voltage designs, current sensing may be performed almost anywhere in the circuit (e.g., wherever the circuit designer deems it appropriate to sense current, including at the low-side, high-side, floating, etc.). With high-voltage designs, however, sensing current at the high-side or at floating locations may involve using complicated and power-hungry circuits, which may jeopardize the practicality and/or the performance of the design. Low-side sensing may, therefore, be desirable to sense current and properly control a high-voltage converter.
It may be desirable to implement low-side sensing using a low-side switch topology in high-voltage environments. Implementing a high-voltage controller in a low-side switch topology may also allow for the use of N-channel MOSFETs, rather than P-channel MOSFETs, for switching, which may be desirable for a number of reasons. One reason is that N-channel MOSFETs may be substantially smaller and less expensive than P-channel MOSFETs. Another reason is that N-channel MOSFETs may be available with voltage ratings in excess of 1,000 volts, while P-channel MOSFETs may typically be available in lower voltage ranges. Yet another reason is that driving a high-voltage P-channel MOSFET may require a level shifter, which may increase the complexity and power demands of the circuit.
FIGS. 2A and 2B show two simplified schematic views of high-voltage buck converters 200 implemented with low-side switch topologies. Both circuits show a high-voltage source (at terminals 206 and 208) generating a high-voltage DC bus (between terminals 202 and 204). For example, the DC bus may be generated by a DC source 210. Each high-voltage buck converter 200 includes a power switch 230. In some embodiments, the power switch is implemented as an N-channel MOSFET. As shown, power switch 230 is configured as a low-side switch tied to the negative DC bus voltage rail 202. A gate driver module 220 generates a switching signal (e.g., a square wave) that drives power switch 230, such that power switch 230 opens and closes as a function of a duty cycle and a switching frequency.
Operation of the high-voltage buck controllers 200 may be similar to that of the low-voltage buck controllers 100 described with reference to FIGS. 1A and 1B. In each cycle, when power switch 230 is closed (e.g., the switch is ON), current flows from the positive DC bus voltage rail 202, through an inductor 216, and a load 250, connected in series. As current flows through inductor 216, stored energy is built up in inductor 216. In each cycle, when power switch 230 is open (e.g., the switch is OFF), the connection between inductor 216 and the negative DC bus voltage rail 204 is interrupted. A diode 214 allows the energy stored in inductor 216 to flow to the load 250, demagnetizing inductor 216. Various embodiments may operate in “continuous” or “discontinuous” modes, as described above. Further, in particular embodiments, one or more capacitors 212 are provided, for example, to reduce voltage and current ripple to the load 250.
It is worth noting that the embodiments of FIGS. 2A and 2B may exhibit different electrical characteristics. According to the embodiment of FIG. 2B, current may flow directly from the positive DC bus voltage rail 202 into the load 250. However, according to the embodiment of FIG. 2A, current flowing from the positive DC bus voltage rail 202 into the load 250 must first pass through inductor 216. Depending on a number of factors, the “floating” load 250 topology of FIG. 2A may be more or less desirable for certain applications (e.g., due to differences in parasitic capacitance, etc.).
The operation of the high-voltage buck converters 200 may also be seen in FIG. 2C, which shows a graph 235 of two cycles of the currents through power switch 230 and a graph 217 of two cycles of the currents through inductor 216. During each cycle, it can be seen that the current through inductor 216 ramps up while power switch 230 is closed, and the current through inductor 216 ramps down while power switch 230 is open. It is worth noting that the current through inductor 216 never reaches zero (i.e., the graph is showing “continuous” operation), and the range between the minimum and maximum currents may be relatively small in certain applications (e.g., five- to ten-percent variation).
As described above, it may be desirable in certain applications to sense the current flowing through the load 250. For example, by feeding the sensed current back into the converter 200, it may be possible to regulate the current in the load 250 (e.g., maintain a desired load current). However, it may also be desirable to sense the current in a high-voltage converter at the low-side to allow implementations using low-voltage components. One difficulty with low-side current sensing may be that, as shown in graph 235, each cycle of current flowing through power switch 230 may begin with a large spike (i.e., from IL1=0 to IL1=IPK). This spike may be related to charge or discharge through power switch 230 of parasitic capacitances, for example from power switch 230, inductor 216, and/or a circuit board. In some embodiments, characteristics of the current spike are further affected by the DC bus voltage level (e.g., between rail 202 and rail 204), switching frequency, and/or other characteristics. As such, while low-voltage converters may operate in the context of relatively clean and easy-to-process waveforms, high-voltage applications may operate in the context of parasitic effects and large spikes.
As such, it may be desirable to provide methods, systems, and devices for implementing enhanced functionality in the context of high-voltage switching power converters.